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National Aeronautics and Space Administration, Science Mission Directorate. (2009). Space Weather 101. Retrieved , from Mission:Science website:


Science Mission Directorate. "Space Weather 101" Mission:Science. 2009. National Aeronautics and Space Administration.

The Variable Sun

Image of the sun showing a Coronal Mass Ejection blasting out from the sun.

A widely spreading coronal mass ejection (CME) blasts more than a billion tons of matter out into space at millions of kilometers per hour.

Looking at the sky with the naked eye, the Sun seems static, placid, and constant. But our Sun gives us more than just a steady stream of warmth and light. The Sun regularly bathes us and the rest of our solar system in energy in the forms of light and electrically charged particles and magnetic fields. The result is what we call space weather. The Sun is a huge thermonuclear reactor, fusing hydrogen atoms into helium and producing million degree temperatures and intense magnetic fields.

The outer layer of the Sun near its surface is like a pot of boiling water, with bubbles of hot, electrified gas - electrons and protons in a fourth state of matter known as plasma - circulating up from the interior and bursting out into space. The steady stream of particles blowing away from the Sun is known as the solar wind. Blustering at 800,000 to 5 million miles per hour, the solar wind carries a million tons of matter into space every second (that’s the mass of Utah’s Great Salt Lake) and reaches well beyond the solar system’s planets. Its speed, density and the magnetic fields associated with that plasma affect Earth’s protective magnetic shield in space (the magnetosphere).

A series of images showing giant sun spots observed from March 26 to March 30, 2001

This was the largest sunspot group of this solar cycle as it moved with the Sun’s rotation. On 30 March 2001, the sunspot area within the group (called active region 9393) extended across an area more than 13 times the diameter of the Earth. It yielded numerous flares and coronal mass ejections, including the largest X-ray flare recorded in 25 years on 2 April 2001, the last image in the series. Caused by intense magnetic fields emerging from the interior, a sunspot appears to be dark only when contrasted against the rest of the solar surface, because it is slightly cooler than the unmarked regions.

Most of the time the effects are benign, but when sunspots appear, it is a potential sign of a space weather storm. Sunspots are dark splotches on the Sun caused by the appearance of cooler (4000° C) areas amidst the roiling gases on the surface (6000° C). Space weather forecasters closely watch sunspots because, like high and low pressure systems on Earth, they hold signs of the severity of what’s to come. The solar magnetic field changes on an 11-year cycle. Every solar cycle, the number of sunspots and solar storms increases to a peak, known as the solar maximum. Then, after a few years of high activity, the Sun will ramp down for a few years of low solar minimum.

Solar Storms

Solar flares and coronal mass ejections (CMEs) are two kinds of solar storms. Solar flares appear as explosive bright spots on the surface of the Sun. Flares occur when magnetic energy built up in the solar atmosphere near a sunspot is suddenly released in a burst equivalent to ten million volcanic eruptions. Radiation—including radio waves, X rays, and gamma rays—and electrically charged particles blast from the Sun following a solar flare. The strongest flares occur just several times per decade, while weaker flares are relatively common, with as many as a dozen a day during the Sun’s most active periods.

A CME is the violent eruption of a huge magnetic cloud of plasma from the Sun’s outer atmosphere, or corona. The corona is the gaseous region above the surface that extends millions of miles into space. Temperatures in this region exceed one million degrees Celsius, 200 times hotter than the surface of the Sun. A number of theories attempt to explain the occurrence of a CME. The magnetic fields in the corona are affected by both new fields emerging from below the surface and by the motions of the plasma at the surface, which carry the fields with them. They can become twisted, and thus energized in localized regions, often creating sunspots. Overlying magnetic fields are like a net holding down a hot-air balloon, restraining the plasma and twisted magnetic fields. Tremendous upward pressure builds. Eventually, some of the magnetic loops merge and burst through the magnetic net, creating a CME.

A series of images showing a CME at different times during the ejection.

A spectacular Coronal Mass Ejection (CME) took off from the Sun in the early hours of January 2002, It startied off as a filament eruption seen by the Extreme ultraviolet Imaging Telescope (EIT) onboard the SOHO spacecraft. The LASCO instrument captured images of the CME in progress.

Observations show that a CME travels through the gulf of space at speeds of up to several million miles per hour (up to 2500 km/sec)! A typical CME can compact more than 10 billion tons of plasma into the solar system, a mass equal to that of 100,000 battleships. A CME cloud typically grows to as wide as 30 million miles across. As it hits the solar wind, a CME can create a shock wave that accelerates some of the solar wind’s particles to dangerously high energies and speeds that create radiation in the form of energetic particles. Behind that shock wave, the CME disturbance travels through the solar system, impacting planets, asteroids, and other objects with enhanced plasmas and magnetic fields. If a CME erupts on the side of the Sun facing Earth, and if its path includes the location of Earth in its orbit, the results can be spectacular and sometimes hazardous.

Space Weather Events

A photo of a glowing aurora.

Aurora appear from Earth as shimmering, dancing lights in the night sky. Only 100 years ago did scientists discover that the Sun was ultimately the cause of these mysterious lights.

Earth inevitably gets hit by CMEs from time to time because CMEs occur at a rate of a few times a week to several times per day, depending on how active the Sun is. Fortunately, our planet is protected from the most harmful effects of the radiation and hot plasma by our atmosphere and by an invisible magnetic shell known as the magnetosphere. Produced by Earth’s internal magnetic field, the magnetosphere shields us from 99% of the Sun’s plasma by deflecting it into space. But some magnetic energy is transferred from the solar wind and CMEs to our magnetosphere, often funneling in near the North and South Poles, where the magnetic field lines meet the surface and the magnetosphere is partially open to space.

The flow of energy into our magnetosphere can induce geomagnetic storms, alter Earth’s magnetic field as measured on the ground, and produce the phenomena known as auroras. A lot of energy is being dumped into the Earth’s magnetic system. When stimulated by plasma from the Sun or from the far reaches of the magnetosphere, the electrons, protons, and oxygen ions surrounding Earth become denser, hotter, and faster. These particles produce as much as a million amperes of electrical current. Some of that current flows along Earth’s magnetic field lines and into the upper atmosphere. Also, excited particles inside the magnetosphere can plunge into the upper atmosphere, where they collide with oxygen and nitrogen. These collisions—which usually occur between 40 and 200 miles above ground—electrically excite the oxygen and nitrogen so that they emit light. The result is a dazzling dance of green, blue, white, and red light in the night sky, also known as aurora borealis and aurora australis (“Northern and Southern lights”). Auroras are visible evidence that something electric is happening in the space around Earth.

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National Aeronautics and Space Administration, Science Mission Directorate. (2009). Space Weather 101. Retrieved , from Mission:Science website:


Science Mission Directorate. "Space Weather 101" Mission:Science. 2009. National Aeronautics and Space Administration.